The core parts for MAD land-based missile silos (to soak up the enemy's missiles) and submarines (to ensure a second strike). Planes are largely a diplomatic deterrent inasmuch as they're easy to send out and easy to recall.

But Pyongyang isn't playing MAD. It's playing credible threat. And for a credible threat, you just need missiles. (On land or on subs.) The point is that you raise the stakes of e.g. a Maduro operation to risking Los Angeles.